CD44-mediated adhesiveness of human hematopoietic progenitors to hyaluronan is modulated by cytokines.

Adhesive interactions between CD34+ hematopoietic progenitor cells (HPC) and bone marrow stroma are crucial for normal hematopoiesis, yet their molecular bases are still poorly elucidated. We have investigated whether cell surface proteoglycan CD44 can mediate adhesion of human CD34+ HPC to immobilized hyaluronan (HA), an abundant glycosaminoglycan of the bone marrow extracellular matrix. Our data show that, although CD34+ cells strongly express CD44, only 13.3% +/- 1.1% spontaneously adheres to HA. Short-term methylcellulose assay showed that HA-adherent CD34+ cells comprised granulo-monocytic and erythroid committed progenitors (19.6% +/- 2.5% and 7.3% +/- 1.0% of the input, respectively). More primitive progenitors, such as pre-colony-forming units, also adhered to HA. Moreover, we found that CD44-mediated adhesion of CD34+ cells to HA could be enhanced by phorbol 12-myristate 13-acetate (PMA), the function-activating anti-CD44 monoclonal antibody H90, and cytokines such as granulocyte-monocyte colony-stimulating factor, interleukin-3 (IL-3), and stem cell factor. Enhancement through PMA required several hours, was protein-synthesis-dependent, and was associated with an increase of CD44 cell surface expression, whereas stimulation of adhesion by H90 monoclonal antibody and cytokines was very rapid and without alteration of CD44 expression. H90-induced activation occurred at 4 degrees C and lasted for at least 2 hours, whereas activation by cytokines required incubation at 37 degrees C and was transient. These data, which show for the first time that CD34+ HPC can directly adhere to HA via CD44, point out that this adhesive interaction to HA is a process that may also be physiologically regulated by cytokines.

Influence of the site of tumor growth on the capacity of a low tumorigenic line of Friend erythroleukemia cells to differentiate.

Friend erythroleukemia cells (FLC) passaged in mice are highly tumorigenic and multiply extensively in the livers of suckling DBA/2 mice without differentiating. In contrast, in vitro passaged FLCs injected intravenously were of low tumorigenicity, multiplied to a limited extent in the livers of suckling mice, and underwent marked differentiation from the proerythroblast to the orthochromatic erythroblast stage in the liver. The presence of characteristic C-type virions budding from the cell surface in various stages of erythroid differentiation served as a marker of the injected FLCs. When the same in vitro passaged FLCs that differentiated in the liver were injected subcutaneously in suckling mice, they formed large subcutaneous tumors consisting of sheets of undifferentiated tumor cells. It is concluded that the tumorigenicity of FLCs depended on the site of tumor growth and that there is an inverse correlation between the tumorigenic capacity and the capacity to differentiate.

Myeloproliferative sarcoma virus: its effects on erythropoiesis in adult DBA/2J mice.

Adult susceptible mice (DBA/2J) infected with MPSV (myeloproliferative sarcoma virus), a defective RNA tumour virus, develop splenomegaly and progressive disruption of the haematologic system culminating in death. The present study was specifically directed toward determining the effects of the virus on erythroid differentiation. Early and late precursor cells (erythroid burst-forming units; BFU-E and colony-forming units; CFU-E, respectively) were evaluated by the ability of bone marrow and spleen cells to form colonies of fully differentiated erythroid cells in vitro. MPSV caused substantial modification of both the BFU-E and CFU-E populations in the bone marrow and spleen of infected animals. Changes were detected in the CFU-E population preceding any significant increase in spleen weight. In the bone marrow, the proportion of CFU-E cells increased almost twofold by days 5-10 after virus infection but decreased by day 15. In the spleen, CFU-E frequency rose 40-fold by days 10-15 and then declined steadily prior to death. At the peak of CFU-E expansion, a small proportion of the population appeared to be erythropoietin (Ep) independent, although there was no evidence of a complete switch to Ep-independence which occurs in Friend virus-induced erythroleukemia. Dose-response curves showed that none of these data could be explained in terms of a changing responsiveness to Ep. However, evidence is presented that indicates that BFU-E from MPSV-infected animals lose or have a reduced requirement for burst-promoting activity (BPA) relative to normal cells although their progeny still need Ep for terminal erythroid differentiation.

Effects of myeloproliferative sarcoma virus on the pluripotential stem cell and granulocyte precursor cell populations of DBA/2 mice.

The hematopoietic stem cell (CFU-S) and granulocyte precursor cell (CFU-C) populations have been assayed in the spleen, blood, and bone marrow of DBA/2 mice at various times after infection with the myeloproliferative sarcoma virus (MPSV). Beginning between 7 and 19 days after virus infection, the number of CFU-S showed a steady, parallel increase in the blood and spleen, reaching a maximum at both sites by days 25-30. At the maximum, in the spleen the concentration of CFU-S was 10 times greater than that in the blood, and the total number of CFU-S was over 100 times greater than that of normal animals. During the same period, in the bone marrow the number of CFU-S decreased to one-half of normal. Nevertheless, the CFU-S from MPSV-infected animals differentiated normally in the spleens of irradiated, normal recipient mice (except for some hyperplasia of the erythroid component of spleen colonies). The CFU-C content of the bone marrow, spleen, and blood paralleled the CFU-S content of these organs: The CFU-S and CFU-C populations changed almost synchronously after MPSV infection. In the terminal stage of the MPSV-induced disease, a variable proportion of the CFU-C population acquired the ability to differentiate in the absence of added colony-stimulating factor.